The present invention relates to a system and a method for controlling the air-fuel ratio in an internal combustion engine, and more particularly, to controlling the air-fuel ratio such that NOx and hydrocarbon emissions are reduced.
Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream three-way catalyst. The LNT, like the upstream TWC, stores exhaust gas constituents such as, for example, nitrogen oxides, NOx, when the engine is operating at a lean air-fuel ratio, and reduces (purges) them when the engine is operating at a rich or stoichiometric air-fuel ratio.
Because continued lean operation will ultimately saturate the LNT with NOx, the prior art teaches periodically varying the air-fuel ratio from a nominally lean setting to a rich setting, during which stored NOx are released from the LNT and reduced by the available hydrocarbons and carbon monoxide in the enriched operating condition. However, during the purge, some of the exhaust gas constituents, such as NOx and HC are not reduced, and are emitted into the atmosphere.
The inventors herein have recognized that the amount of NOx and HC released during a purge is a function of the temperature of the LNT and the air-fuel ratio of the purge mixture. In other words, the inventors have recognized that when the LNT operating at lower temperatures, the purge air-fuel ratio has little effect on NOx emissions, however, HC emissions increase with richer purge air-fuel ratios. Further, the inventors herein have recognized that at higher LNT operating temperatures, NOx emissions reduce with richer purge air-fuel ratios, while HC emissions increase.
In solving the above problem, a system and a method are provided for decreasing emissions of exhaust gas constituents by selecting an optimum purge air-fuel ratio based on the operating temperature of the exhaust gas aftertreatment device.
In carrying out the above solution, features and advantages of the present invention, a system and a method for improving a conversion efficiency of an exhaust gas aftertreatment device coupled downstream of a lean-burn internal combustion engine during a NOx purge, include: providing a base rich air-fuel ratio of an exhaust gas mixture entering the device; adjusting said base air-fuel ratio so as to increase a first exhaust gas mixture component conversion efficiency when the device is in a first operating temperature range; and adjusting said base air-fuel ratio so as to increase a second exhaust gas mixture component conversion efficiency when the device is in a second operating temperature range.
An advantage of the present invention is improved emission control, due to the fact that selecting the most appropriate air-fuel ratio based on the temperature of the LNT reduces NOx and HC emissions during purge.